1. Field of the Invention
This invention relates to a fuel control apparatus for an internal combustion engine and, more particularly, to a fuel control apparatus for an engine for detecting the pressure in an intake pipe of the engine and controlling fuel injection on the basis of the detected pressure value.
2. Description of the Prior Art
It has hitherto been the practice to detect the pressure (intake negative pressure) in an intake pipe of an engine and control fuel injection, spark timing, etc., of the engine in response to the detected pressure signal. However, certain problems have been encountered since such a negative intake pressure includes a pulsating variation, so a surging phenomenon is generated if control operations are effected directly in response to the detection of the negative intake pressure, thereby resulting in incorrect control of various functional quantities.
To solve such problems, it has been known that a fuel control apparatus may be provided, in addition to a pressure sensor for sensing the pressure in the intake pipe, with a smoothing circuit for smoothing the output of the pressure sensor in order to remove any pulsation in the intake pressure and detect the mean value thereof, thereby preventing the occurrence of the surging phenomenon.
In this kind of fuel control apparatus, a pressure sensor which detects the pressure in the intake pipe (negative intake pressure) is provided in a position downstream of a throttle valve in the intake pipe which supplies the intake air to the engine. The detected signal output from the pressure sensor is supplied to the smoothing circuit comprising, for example, a filter, where the pulsation component is removed. The intake pressure signal from which the pulsation component has been removed by the smoothing circuit as described above is supplied to a control circuit for controlling fuel injection, etc., of the engine.
Such a conventional fuel control apparatus, however, has certain drawbacks in that, because of the smoothing of the pulsation contained in the intake pressure signal which is undertaken with a view to detecting a mean value thereof, there is response lag in the detection of the intake pressure during operation of the engine in a transitionary phase, i.e. during acceleration or deceleration, and in the case of a fuel injection control such a response lag results in a fuel undersupply which will cause the engine to stall.
This will be described below in more detail. Assuming that the opening .theta. of the throttle valve varies as time elapses as shown in FIG. 1(a), the pressure Pb.sub.AD in the intake pipe, i.e., the output signal of the pressure sensor varies as shown in FIG. 1(b). This pressure value Pb.sub.AD contains a ripple, as shown. When the pressure value containing the ripple is processed by the smoothing circuit, a smoothed pressure value Pb.sub.F as shown in FIG. 1(c) is obtained. This smoothed pressure value Pb.sub.F has a response delay relative to a variation in the intake pipe pressure Pb.sub.AD corresponding to a change in the actual throttle opening .theta. at the time of acceleration or deceleration, so that the air fuel ratio will become lean during acceleration and rich during deceleration, as shown in FIG. 1(d), and thus the engine performance during acceleration will be lowered and shocks will be generated due to the rough running of the engine during deceleration.
Japanese Patent Application Laid-Open No. 24829/1983 discloses a fuel control apparatus for an engine which is intended to solve the problem that the smoothed pressure value has a response lag relative to any change in the intake pipe pressure corresponding to a change in the throttle opening, and is arranged so that the function of the smoothing circuit which is designed to smooth the output of the pressure sensor is reduced or eliminated when the engine is in a transitionary condition, thereby improving the response of the intake pressure detection during such a transitionary state and inhibiting, as much as possible, the generation of surging due to pulsation of the intake pressure.